1. Field of the Invention
The invention relates to a separator unit and a fuel cell stack.
2. Description of the Related Art
A fuel cell that discharges no harmful substance while exhibiting high power generation efficiency has been put into practice as a power generating system for industrial and household use or as a power source for a satellite, spacecraft or the like. Recently, the fuel cell serving as the power source for vehicles like passenger cars, buses, and trucks has been under development. The fuel cell of alkaline solution type, phosphoric acid type, molten carbonate type, solid oxide type, direct methanol type and the like may be employed. However, the proton-exchange membrane type fuel cell is mostly employed as it reacts at a relatively low temperature and is advantageous to downsizing.
In the aforementioned fuel cell, a membrane electrode assembly (MEA) formed by joining two gas diffusion electrodes and a solid polymer electrolyte membrane interposed therebetween is employed. One of the above gas diffusion electrodes serves as a fuel electrode (anode electrode), on which hydrogen gas is supplied as a fuel, decomposing an hydrogen molecule into hydrogen ions (protons) and electrons so that the hydrogen ions permeate the solid polymer electrolyte membrane. The other gas diffusion electrode serves as an oxygen electrode (cathode electrode), on which air is supplied as an oxidizer so that oxygen in the air is combined with the hydrogen ions and electrons, generating water. The above-described electrochemical reaction generates an electromotive force.
The proton-exchange membrane type fuel cell has a stack structure in which a separator is provided on an outer side of the MEA to form a passage for supplying reactant gas including hydrogen gas as the fuel gas, and oxygen as the oxidizer gas. The separator serves to prevent permeation of the reactant gas into adjacent MEAs in the stack direction, and to collect the generated electric current that is brought to the outside. A plurality of unit cells each having the MEA and the separator are stacked to form a fuel cell stack.
In the fuel cell system, the heat energy substantially equivalent to the generated power in each of those cells is generated through the electrochemical reaction. Especially the proton-exchange membrane fuel cell operated at a low temperature is provided with a cooling unit that prevents excessive temperature rise in the respective cells as disclosed in Japanese Patent Application Publication Nos. JP-A-8-306371 and JP-A-10-340734.
In the generally employed fuel cell system as mentioned above, a temperature distribution may occur with respect to the stack direction of the unit cells, thus increasing the temperature of the electrode. In this case, quantity of water that dissipates outside the MEA through the electrode is likely to become excessive, thereby reducing the humidity within the MEA and thus reducing the power generation efficiency in each of the cells.